The present invention relates to an industrial and inexpensive method of producing powder and emulsion of thermoplastic resin.
There are several methods as the methods of producing powder and emulsion of thermoplastic resin. Production methods of powder represented by mechanical pulverization and chemical pulverization and production methods of emulsion represented by emulsification polymerization, suspension polymerization and post-emulsification are included.
In the production methods of powder, the mechanical pulverization can be said to be an inexpensive production method, but has drawbacks of relatively large particle diameter and poor particle shape.
Whereas, the chemical pulverization is said to give relatively small particle diameter and also good particle shape, but has drawbacks of high cost and potential of environmental pollution because of the use of large quantities of organic solvent.
Moreover, in the production methods of emulsion, polymers represented by ethylene-vinyl acetate copolymer, which can be obtained through the emulsification polymerization or suspension polymerization, have generally only low ethylene content. Polymers with ethylene content over 50 wt. % require a high-pressure process of reaction pressure of several hundreds kg/cm.sup.2 or more, which is substantially difficult in practice from the aspects of cost and safety, and the upper limit of ethylene content of ethylene-vinyl acetate copolymer obtainable by such methods is said to be 40 wt. %.
On the contrary, the post-emulsification method is excellent as a method of emulsifying high-olefin content polymers, since the thermoplastic resin being usually solid is dispersed into water in the subsequent process.
With respect to the post-emulsification process, too, cases with using solvent and without using it are conceivable. The former is a method wherein a solution dissolved thermoplastic resin into a suitable organic solvent is mixed and emulsified with an aqueous solution of emulsifier and then the solvent is collected. According to this method, the emulsification of thermoplastic resin with relatively high molecular weight is possible, but the collection of solvent is difficult also technically, hence this can be said to be a production method of low productivity and high cost. Also, this is unpreferable from the point of environmental problem.
With respect to the latter method, a method wherein a composition of polyolefin and carboxyl group-containing polyolefin is brought to a molten state and a basic substance and the composition are dispersed into water at 140.degree. to 300.degree. C. (for example, Japanese Patent Publication No. Sho 58-42207) is proposed. This is excellent in the point that the physical properties of resin are not injured because of no use of emulsifier, but specific in the point that, unless carboxyl group-containing polyolefin is used, sufficient effect cannot be expected. Moreover, upon producing fine particles, the effect of melt viscosity is significant and the process requires high temperature and high pressure, thus this method is unsuitable for emulsifying relatively high-molecular weight resin and disadvantageous also from the aspect of energy. Further, as an another method, a method wherein an olefinic resin and an aqueous solution of partially saponified poly(vinyl alcohol) are kneaded moltenly in a multiaxial extruder to produce an aqueous dispersion (for example, Japanese Patent Publication No. Sho 62-29447) is proposed. In this method, the partially saponified poly(vinyl alcohol) to be used is restricted to one with high polymerization degree from the fact that, unless an aqueous solution of partially saponified poly(vinyl alcohol) with high viscosity is used, fine particles cannot be obtained. Moreover, the preparation of aqueous solution of partially saponified poly(vinyl alcohol) with high viscosity is a difficult work and feeding this into extruder is also accompanied with difficulty. Furthermore, in this method, unless the aqueous solution is fed into the compression zone of multiaxial extruder, fine particles cannot be obtained. The screw of multiaxial extruder having compression zone is very special and not used universally.
Different from such conventional methods, the invention lies in that, only by supplying water on the way of general purpose biaxial extruder, very stable aqueous dispersion of thermoplastic resin can be produced continuously, inexpensively and easily without using specific emulsifier or dispersant.
As a result of extensive investigations in view of the status quo aforementioned, the inventors have found that, by using a thermoplastic resin composition moltenly mixed general purpose poly(vinyl alcohol) with thermoplastic resin, very stable aqueous dispersion of thermoplastic resin can be produced continuously, inexpensively and easily, leading to the completion of the invention.